Complex Nb-Ta-Ti-Sn oxide mineral intergrowths in the calandria pegmatite, Cañada del Puerto, Córdoba, Argentina

Abstract

The assemblage of Nb-Ta-Ti-Sn oxide minerals in some zoned Devonian granitic pegmatites at the La Calandria mine, Canada del Puerto, shows complex intergrowths of achalaite, ferrotitanowodginite, Ta-rich rutile, columbite-(Mn), ixiolite, and fluorcalciomicrolite. The chemical composition of these phases is variable, but in general the minerals show slight predominance of Nb over Ta and a clearly defined predominance of Fe over Mn, with minor participation of Ti, Sn, and W. The minerals in this assemblage are not in equilibrium and represent magmatic and subsolidus phases that are distinguished texturally and chemically. The primary magmatic stage of mineralization possibly crystallized ixiolite IþTa-rich rutile I in the outer zones of the pegmatite and, less commonly, local columbite-(Mn) in the inner part. Subsolidus unmixing of ixiolite I produced achalaite and ferrotitanowodginite I þ tantalian rutile II. Contemporaneously, Ta-rich rutile I locally exsolved ferrotitanowodginite/achalaite IIþcassiterite. A localized Ca-F-rich hydrothermal overprinting event transformed Ta-rich rutileI to Ta-rich rutile III þ ferrotitanowodginite/achalaite IIfluorcalciomicrolite. Furthermore, the hydrothermal overprint produced peripheral transformation of columbite-(Mn) to ixiolite II and widespread distribution of luorcalciomicrolitethroughout the assemblage.